Artificial line



Jan. 13, 1931,.

A K. s. JoHNsoN 1,788,526

ARTIFICIAL LINE Filed July l1, 1928 ATTE/w/AT/o/v o ATTE/Vanna 7:0.

0 FRmufA/cy CRS.

Patented Jan. 13, 1931 MTE STATES@ PATNTPOFFICzE,

KENNETH sJOnNsoN,l OE JERSEY CITY, NEW JERSEY, AssIeNjoR To BELL TELEPHONE LABORATORIES; INCORPORATED, OE NEW YORK,N. Y., A .CORPORATION OE YNEW YORK i ARTIFICIAL LiNE Application `lle. July 11, 1928. Serial No. 291,803. A

This invention relates to artificial lines and more particularly to artificial lines for simu"- latingthe impedance o" a line having uni-V formly distributed constants. u

The invention provides'artiicial lines'of4 the recurrent type having the property that their characteristicimpedances are similar tov and by` proper adjustment oi the values of the` component impedances, may be made equal Vto the characteristicL impedance of a given smoothline. f

`Such lines may be used for balancing a smooth line, as in the operation of telephonek repeaters,`oi thedupl'ex telegraph systems, and a highly accurate" balance maybe Vattained, provided that both lines are of 'such length or are so terminated, that Wave reflection kfrom their remote ends Vis negligibly small. Y

The inventionk also provides artilicial lines vof the type described above, the attenuation characteristics oi which maybe varied With-v in Wide limits, thereby enabling the lines to be used as attenuation equalizers, or for the simulation of both the impedance andk thel propagation characteristics of a smoothl line.A

These objects are attained by recurrent line structures in which resistive impedances `and complex, resistive and reactive, impedances alternate with each other in series and shunt relation with respectto the line.V The compleX impedance branches are of the three-element type, comprising tivo resistances and one reactance, Whichmaybe either ind-uct-l ance or capaclty. ln certain embodnnents of the invention the complex impedances constitute'the series impedances of the line, and in other embodiments they are used- Composite structures as shunt impedances. may also be used in which portions of both types of lines are used in combination. Variation of the attenuation characteristic, Without affecting the characteristic impedance, is made possible by the `use ol three-elo ment impedance branches in the line con struction.

The invention Will be better understood from the following detailed description and by reference to the dra-Wing, of Which Figs.

. tion.

l, 3, 5 and 7 represent Asectionsorn previously known artificial lines orsimulatlng the characteristic impedance o nsmoothl1nes. Since Athey form the basislfrom which" the various forms of the ypresent invention are derived,y it is convenient to describe'themas 4protor u p u l types. Figs. 2, 4,. 6 and 8 represent artiiicial the characteristic impedance cfa` smooth line,

lines embodying the invention. Fig. 9 shows a composite networkjembodying the inven- F ig. l0 illustratesV attenuation characteristics of thecomp'osite 'network of Fig. 9 and its component'parts. Figzfl'l illustrates variations in attenuation kcharacteristics 0b tainable in the networks ofthis invention.

A smooth line is one Whose impedance ele-v ments are uniformly distributed inv character,

-v and is exemplilied by nonfloaded open -Wire linesand cables. vThe characteristicimpedj. ance of a smooth line is the impedance ineasf ured at the kinput terminals of a line of inn L, nnite length, or of linlte length if the remote end is terminatedbyan impedance Which is equal tojthe characteristic impedance at all frequencies. f The characteristic impedancel varies With frequency and is expressed by the formula:

R -l-jwL.

Where ZL=the characteristic impedance,

`mile, G=leakage conductance in ohms per loop mile,` 4. C=capacity inmicroi'arad per loop mile,

wl=21r times the frequency? lThe artificial lines of the presentinvention consist of lumped impedance structures'of the series-shunt type, the characteristic impedances of which may be made to have the value. expressed by equation (l) Consider pedances equal to half the impedance of full series arm. Similarly, the sections of Figs. 3 and 7 are saidV to have mid-shunt terminations since the terminating arms are shunt admittances equal to half the admittance of a full shunt arm., |The impedance measured Y at the input terminals; of thesestructures are designated respectively, mid-series impedance and mid-shunt impedance. The elements of the structure Fig. l are!de'tei-minedI from thesmooth:line-constants by the formular-z The elements ofthe section o;Fi`g. rarever.A

'VP-fesse@ bythe fOrmulae:

i These networks are suitable-.for simulating 5,5;k 'Fha elements?. ofthe; structures: illustrated! in the imperi-aimes of lines` for'which-RC- LG' positive, that: is, for-"Which In Fig. 5, the elementsothe structure can be expressedbytheformul: p

For vfurther information relative to these structures reference is made to British Patent No. 223,336, of October 23, 1924.

The sections of lines shown in Figs. 2, 4, 6 and 8 which represent forms ot the present ivention are derived from the structures of Figs. 1,;3, 5 and 7 respectively as prototypes. As indicated in the drawings, the values of the elements vof the derived sections are obtained i'romthe elements of the respective prototypesby the applicationeta-numerical factorm. Consequently they are designated l17a-derived structures of the prototypes. A feature of any of these m-.derived networks is that its characteristic impedance is the same as that of itsprototype for any value of m, Whenthe impedance ismeasiired either atinid-'seriesor l L mid-'shunt according to the terminationsao'l thev prototype. Thus the rind-series.characa teristic; impedance of the network. in Fig; 2 isthe same as the midfseries .characteristic im-v p edance of the networkinf Fig. 1'.- Likewise j it, "follows that the4 mid-shunt characteristic impedancejof the prototype; of 3 and of the m.-d'ciived typev of Fig-"Llare equal andr simulate the smooth line characteristic impedance at all frequencies. The same;reason; ing applies tothe structures` illustratedf` in Figsg. 5 6, 7 and. 8. `Ansinspectifon ofthe values of' the elements, of' the viz-derived. net.-

Works shows that they arefphysically realkizable when the value of m satisfies the con-A dtOn. 0 ,m 1. Y .l

Whilethef characteristic impedances of the lllt-derived types are the same' as'those of.

theirz respective,prototypes,.there is av difterence between the attenuation characteris: tics;` The prototypes all. have, infinite at'- tenuatio'nfeither at Zero or at infinite` frequencyfwhereas the m-derived types do, not.

Fig. 1D illustrates the type of? variationof they attenuation with. frequency in. the, net: Works oijtlie` invention. The ordinates repnesentattenuation.measured in transmission units` The transmission unit,l T, is a. logarithmic ratio of thev power attenuationA intlenetwork. A treatment ofthis unit may. beounglgin Chapter II of` Transmission Circuits for Teleplionic Communication by: K1 S.' Johnson published by D. Van Nose iio trandCompany, NewYork. vCurve a repre-` sents a characteristic of the structure ofd either Fig'. 2`0r Fig.v 8; Curve?) is representa.,- tive ofthe type of'attenuation characteristic Vobtainable in1 the structures shown in either Figs. 4 or 6:V Y n c When the networks of the invention are used for simulating or for balancing the-im'- pedance of a given smooth line, the simula,- tion orthe balance can be effectedto any desired .degree of accuracy by the use of av re1- atively small number of sections, providedl the overall attenuation in the simulating line is sufficient'y torender the eiilect oit'. reflection `atVV its end remote from theV input terminals,

negligibly small. On account of their great?` cies than at high, Whereas the structures of Figs. 2 and 8 are better at high frequencies. By combining a section of the type shown in Fig. 2, however, with one of the type shown in Fig. 4l, a composite section isobtained in which the attenuation is nearly uni format all frequencies. Such a network is shown in Fig. 9 and the attenuation characteristic of the composite section is shown by curve c of Fig. lO. An artificial line made up of composite sections of this type is therefore effective to simulate or balance the impedance of a smooth line with a nearly uniform accuracy at all frequencies, and for a given degree of precision in the simulation it will require a total number of single sections less than half the number required in a. line using sections of only one type.

The networks of Figs. 4 and 6 provide attenuations which diminish as the frequency increases, this being a variation of an inverse type to the variation of the attenuation in a smooth line. These networis may therefore be used to affect an equalization of the attenuation in a smooth line, and moreover, may be inserted at any point therein without causing reflection loss. Byjthe choice of a suitable value of the factor m, or by the use of a plurality of sections for which 'the factor has different values, the compensation of the attenuation variation may be effected with a satisfactory degree of precision over any given range of frequencies. attenuation characteristics-for various values of m, a family of curves similar to curve 7 of Fig. l0 is readily obtainedv by means of which the proper selection andcombination of sections to give a desired equalization is facilitated.

The effect of the factor m on the attenuation characteristic can be determined from an examination of the propagation constant for any series-shunt type of line which is eXpressible as Z2, 1`/i +4 Z where P=the propagation constant per section of line,

Z1=the impedance of a full series arm of the section, y

Z2=the impedance of a full shunt arm of the section. The propagation constant, l), is a complex quantity which may be written:

Where B, the imaginary component measures the phase shift per section, and A, the real component, measures the attenuation per sec- By plotting thel tionif Consider the network of Fig;2in1/WhichV at-zero frequency; A. f

" Z1-#mat een At infinite frequency, y y

..1 When m=o, =o.

Freni Equati on 14 it is seenthat the propaga.- tion constant andvlience the attenuatiomincreases with l p Thus-for m: l, the attenuation characteristic of the structureof Fig. 2 is represented by a roo curve having a finite value at zero frequency and rising to an infinite value atinfinite frc quency. This is represented in Fig'. 11 by the full line curve (mil). The attenuation for l1n=0 is represented by the base line. For values ofi/mJ ranging between 0 and l, there will be a family of curves represented bythe full lines. These curves are also representav tive of the attenuation characteristics of the section in Fig. 8. It can be shown in a sim ilar manner that the attenuations of the structures in Figs. i and 6 are as represented by the family of dotted line curves. It is noted that for the value of m=1, the m-derived networks reduce to the prototypes. A further use of the composite section of Fig. 9 is to insert into a smooth line varying amounts of nearly uniform attenuation. The attenuation can-be increased by increasing the number of composite sections.

What is claimed is: l. An artificial line of the recurrent type having acharacteristic impedance equal to that of a uniform smooth line at all frequencies, comprising resistance branches `alternatlll 0 v ingin series-shunt relation withbranches having three impedance elementsatwo of which are resistances and the third a reactance.

2. An artificial lineo't' the` recurrent type having a,` characteristic.impedance, equal to that of a uniformsmoot'h lineat all' frequencies, comprising a symmetrical three-brauchtV section,V two of the branches of said: section consisting;v of equal resistances andthe third being a three-element impedance consisting of tworesistances and a reactance. K 3. An impedanceA network for simulating the impedance of a given smooth line7 com- Y prising a pair of sections each of the type described in claim l, one having attenuation which increases, the other, which decreases with increasing frequency 4. In combination with a smooth line, an

i impedance network .comprising two symmetrical three-branch sections of the type described in claim 2one offwhichl has two equal y resistance branches disposed in shunt to the line, and the other of which has two equal resistance branches disposed in series, each section having a characteristic impedance equal tothat of said smooth line.

5. An artificial line having a characteristic impedance equal toithat of a luniform smooth line at all frequencies, comprising a pair of symmetrlcall sections, one of. which has a series arm composed of tworesistan'ces'and .a reactance and two shunttarms composed of equal resista-mees, the other of which has 'ay shunt' arm composed of twoV resistances and:

a reactance and two series arms composed'of" equal resistances.

In witness whereofI hereunto subscribe Y my name this 10th day of July, 1928.

KENNETH s vJorrNsoNf.

. *mesme 

